I actually just had this same issue. I had changed the minimum to 1 night for an exception stay, and it would not change back to the 2 night minimum. Even after trying all the solutions posted in the forum, nothing worked.
I just received a notice from support there is an additional setting that needs to be adjusted. There is a minimum stay by check-in day that apparently defaults when you change the main setting.
Steps to reset this are below. This does work!! And you don't have to resort to setting specific time periods.
Steps:
Using your computer:
1. Open your listing
2. Select Calendar
3. Find Availability in the right hand sidebar
4. Click the number for Minimum nights
5. Click Customize by check-in day
6. You will see the minimum nights set up per night is 1 from Sunday to Monday (your number may vary)
7. Click the number for each day and remove
8. Click Save
After all the steps, kindly double-check your listing, and test to try rebooking your place and should be the correct nights minimum.